A fiber-reinforced composite material containing a thermoplastic resin as a matrix (to be sometimes referred to as “FRTP” hereinafter) is widely known. Generally, FRTP is a prepreg obtained by impregnating reinforcement fibers in the form of dispersed pellets, unidirectional pellets, a single strand, a unidirectional sheet (UD sheet), a woven fabric or a non-woven fabric with a resin. A prepreg is processed into a structural member, various parts, etc., by injection molding, press molding, in-mold forming, filament winding molding, etc.
When FRTP is produced, however, the following points are to be noted. That is, since a thermoplastic resin generally has a high viscosity, its adhering amount to fibers is large and it tends to adhere non-uniformly, so that it is difficult to impregnate each one of fibers with a thermoplastic resin. Further, a gas generated by heating a thermoplastic resin is difficult to dissipate to an outside, and the adherence of fibers and a resin is liable to be insufficient.
FIG. 5 shows one example of an apparatus for impregnating a continuous fiber by a melting method using external heating (parenthesized numerals indicate reference numerals in FIG. 5). This apparatus has a vessel (11) containing a molten thermoplastic resin (12), and is constituted to ensure that a continuous fiber bundle (1) is immersed in the vessel (11) to impregnate the continuous fiber bundle (1) with the molten resin (12). In this case, it is arranged that the viscosity of the thermoplastic resin is decreased by heating (external heating) with pinch rollers (14) so that the fiber bundle may be easily impregnated with the thermoplastic resin.
In this method, it is required to increase the temperature of the resin in the vicinity of the fiber for decreasing the viscosity of the resin in the vicinity of the fiber and improving the adherence to the fiber. Since, however, heat in this method is transmitted in the order of the resin→an interface between resin and the fiber→the fiber, it is required to increase the temperature of the entire resin for increasing the temperature of the interface, and as a result, the entire resin comes to be exposed to a high temperature and is thermally deteriorated.
Further, various proposals have been made with regard to a method of improving the adherence of a resin and a fiber and improving the mechanical properties of FRTP. For example, the following proposals are examples.
Patent Document 1 proposes a process for continuously producing a prepreg sheet, which comprises the steps of continuously taking up a fiber bundle while a tension is exerted along a curved surface press-widening the above fiber bundle, drying the thus-press-widened fiber bundle and impregnating the dry fiber bundle with a molten resin. The above process not only overcomes the problem of a residual solvent but also overcomes the insufficiency of press-widening of the fiber bundle in a dry method. In this method, however, it is difficult to control the amount ratio of the resin and the fiber, and it cannot be said that sufficient adherence can be accomplished.
Patent Document 2 discloses a process for producing a prepreg, which comprises impregnating a sheet-shaped continuous carbon fiber widened/opened unidirectionally in parallel with a solution of a resin different from a matrix resin and compatible with the matrix resin and bonding the impregnated carbon fiber without drying to a sheet-shaped resin that is the matrix resin. Since this method uses, as glue, a resin solution compatible with the matrix resin, it has a defect that a residual solvent degrades the performance of a composite material.
Patent Document 3 proposes a method for producing a prepreg, in which a sheet-shaped continuous carbon fiber widened/opened unidirectionally in parallel is pressed to a thermoplastic resin film in a molten state to impregnate the carbon fiber with the thermoplastic resin. This method cannot yet be said to give sufficient adherence.
Patent Document 4 proposes a method for continuously producing a fiber-reinforced plastic, in which a fluid plastic is separated into many flows and the many flows are caused to collide with a fiber material by ejecting them perpendicular to the fiber material. When this method is applied to a thermoplastic resin, the viscosity of a molten thermoplastic resin is too high, so that it is required to prepare a solution or dispersion thereof. It cannot be said, either, that the above method fully overcomes the problem of a residual solvent.
Patent Document 5 discloses a method for producing a prepreg, which comprises removing a sizing agent of a fabric with a solvent in a prepreg containing the fabric as a reinforcing material and the thermoplastic resin as a matrix, then drying the prepreg, stacking a thermoplastic resin film and pressing the resultant set under heat. Even in this method, however, it cannot be said, either, that the adherence of the resin and the fabric to each other is sufficient.
Patent Document 6 discloses a laminate molding method in the stamping forming of a fiber-reinforced prepreg containing polypropylene as a matrix resin, in which an additive for improving a dielectric loss coefficient is incorporated into the matrix resin and the prepreg is heat-melted by means of microwave. In this method, however, the additive in the matrix resin is heated by the microwave, and the fiber and the resin in the vicinity thereof cannot be selectively heated. It is therefore required to heat the entire matrix resin to a high temperature for decreasing the melt-viscosity of the resin close to the fiber to a desired melt-viscosity, so that the entire matrix resin may be thermally deteriorated.
(Patent Document 1) JP-A 57-56220
(Patent Document 2) JP-A 58-162317
(Patent Document 3) JP-A 59-14924
(Patent Document 4) JP-A 61-286107
(Patent Document 5) JP-A 63-54441
(Patent Document 6) JP-A 64-34733